The present invention relates to a control system for controlling the operating speed of a can machine. As used herein, a "can machine" is a machine for receiving cans or empty can shells without tops for filling the cans, applying a top and sealing it, or labeling the can. In an automated can production line, a number of machines are arranged to perform sequential operations on the cans which are normally conveyed from one machine to another along a conveyor. If the upstream machine is processing cans faster than a downstream machine is capable of processing the cans, there will be a net build-up of cans in the conveyor. Similarly, if the downstream machine processes cans faster than an upstream machine is capable of producing them, the downstream machine will quickly begin to have "misses", thereby reducing efficiency.
It has long been a practice to sense cans in a queue, such as a single file queue and to speed up the downstream machine or slow down the upstream machine if the cans are accumulating in number in the queue or, conversely, to slow down the downstream machine or speed up the upstream machine if the number of cans in the queue is diminishing.
The sensing may be done mechanically, with feelers and limit switches, or it may be done electrically. A typical electrical system for sensing the number of cans in a single file queue is to provide a plurality of proximity sensors along the input conveyor in which the queue is formed. For example, proximity sensors may be located at six inches, twelve inches, eighteen inches, and twenty-four inches from the input of a machine. If there are enough cans only to fill the conveyor up to the nearest sensor, then only that sensor generates a signal to control logic circuitry for performing a speed control function. If the two nearest sensors have cans adjacent them, then separate signals are transmitted to the controller for performing a speed control function, and so on. This type of sensing is acceptable, in some cases, but it does not provide a single signal representative of the total number of cans in the queue. If such a signal were provided, it would have the advantage of permitting a more continuous range of control of the variable speed controllers for the machines. Such variable speed controllers are well-known and capable of operating in response to much smaller increments of signal change than are permitted by the discrete sensors mentioned above.
Moreover, in the case of a conveyor having room to permit more than a single file of cans, the problems of sensing the total mass of cans present and providing finer control increments is even more difficult with existing commercial techniques.